The invention relates to collimators, and more particularly relates to focussing collimators for use in single photon computerized tomography, or SPECT. In its most particular sense, the invention relates to focussing collimators for use in SPECT examinations of the head.
U.S. Pat. No. 4,670,657 discloses an astigmatic collimator which is well adapted to such examinations. Cone beam collimators are also well adapted to such examinations. However, as is set forth in U.S. Pat. No. 4,670,657, it is advantageous to incline the sensitive crystal surface with respect to the axis of rotation during a SPECT (single photon emission computerized tomography) examination. This reduces the variation in the gap between the patient's brain and the sensitive crystal surface.
When the camera head is so tilted, and especially at inclinations of 25 to 30 degrees, it has been found that artifacts are introduced in the reconstructed tomographic image. These artifacts are undesirable.
One object of the invention is to provide a focussing collimator which may be inclined with respect to the axis of rotation during SPECT without causing excessive artifacts to arise in the reconstructed image.
Another object is to improve on existing cranial scanning techniques and apparatus.
In accordance with the invention, an astigmatic, cone-beam or anamorphic collimator is provided in which the principal ray is offset so that, during rotational camera transaxial SPECT in which the camera head is inclined with respect to the axis of rotation, the transaxial plane bisects the angle bounded by the two radially outermost fan strips (in the case of astigmatic and anamorphic collimators) and the angle bounded by the two radially outermost collimated rays (in the case of a cone-beam collimator).
This reduces the maximum inclination of a collimated ray with respect to the transaxial plane. As a result of this reduction, reconstruction artifacts are reduced to a level at which they are masked by the ambient noise level which is inherent in all scintillation camera systems.